1. Field of the Invention
The present invention relates to improvement of a ferrofluid sealing device for use in, e.g., a spindle motor that drives a magnetic recording medium utilized in the information industry.
2. Description of the Prior Art
A conventional ferrofluid sealing device is constructed as shown in FIG. 7. That is, a pair of pole pieces 1, 2 made of magnetic bodies, each having a shape of a doughnut-like disk, sandwiched between their inner surfaces is a magnet 3 shaped into a doughnut-like disk and having an inner diameter larger than that of either of the pole pieces; the outer peripheral faces thereof are fixed to a housing 6; a shaft 7 made of a magnetic body is inserted to the center holes of the doughnut-like pole pieces; and a ferrofluid (a magnetic fluid) 5 is retained, by a magnetic circuit formed by the magnet 3, in a gap between this shaft 7 and a front end surface 4 of the outer pole piece 1 facing the shaft and located on the side where the ferrofluid is retained. Reference numeral 8 denotes a ball bearing.
In the ferrofluid sealing device described above, not all of the magnetic flux generated by the magnet 3 can flow from the pole pieces 1, 2 to the shaft 7. Some of the magnetic flux leaks when it flows from the vicinity of the inner diameters of the pole pieces 1,2 to the shaft 7, causing the ferrofluid 5, which is injected to and retained in the gap between the shaft and the pole pieces 1, 2, to have a shape bulging axially from the shaft toward the pole piece 1. The bulging ferrofluid splashes outward upon high-speed rotation of the seal device, which brings about inconveniences such as shortened lifetime due to reduction in amount of the ferrofluid 5, or stained surroundings due to the ferrofluid 5 flowing out onto the outer surface of the pole piece 1.